Object location system



Sept. 14, 965 R. K. c. JOHNS 3,205,605

OBJECT LOCATION SYSTEM Filed Feb. 23. 1961 '7 Sheets-Sheet l ROMAN K.C.JOHNS INVENTOR.

zzm mm A 7'TORNE K8 R. K. C. JOHNS OBJECT LOCATION SYSTEM '7sheets-Sheet 2 K c JOHNS INVENTOR. BY/7mw-% ROMAN A TTORNEYJ Sept. 14,1965 Filed Feb. 25, 1961 Sept. 14, 1965 R. K. c. JOHNS 3,205,505

OBJECT LOCATION SYSTEM Filed Feb. 23. 1961 7 Sheets-Sheet 3 TIME DATAINDICATOR LINK I ORIENTATION ORIENTATION CONTROL CONTROL FIG. 3 B2 ROMANK.C. JOHNS BI B2 INVENTOR.

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OBJECT LOCATION SYSTEM Filed Feb. 23. 1961 7 Sheets-Sheet 7 OPTICAL FLATFIG. 8

Bl Tl C X2 XI 0 A =ERROR SIGNAL M ERROR SIGNAL E Y-Yl =ERROR SIGNALPROCESSlNG R Y-Y2 =ERRR SIGNAL A F/G. l0 ROMAN K.C. JOHNS INVENTOR.

BY WWYQMQ ATTORNEYS from earth for each pair of beacons are required.

United States Patent 3,206,605 OBJECT LOCATION SYSTEM Roman K. C. Johns,Winchester, Mass, assignor to Baird- Atomic, Inc., Cambridge, Mass., acorporation of Massachusetts Filed Feb. 23, 1961, Ser. No. 91,194 4Claims. (Cl. 250-203) The present invention relates to the art of objectlocation and more particularly is directed towards a novel method andsystem for the location and tracking of remote objects. More especiallythe invention relates to systems and methods for the location andtracking of extra planetary objects such as orbiting satellite bodies.

Radio devices of various types utilizing the radio frequency section ofthe electro-magnetic radiation spectrum are used for the purpose oflocating and tracking objects in space and determining their orbits.While such devices have the advantage of being operable throughout theday, i.e., for twenty-four hours a day, in the present state of the artsuch systems are inherently incapable of providing the degree ofconsistent accuracy necessary for certain applications.

In geodetic and astronomic work, a consistent accuracy of one to twoseconds (1-2") of arc is required. This requirement implies the use ofat least supplementary optical tracking devices that are operabletwenty-four hours a day to enable continuous calibration of radiodevices while the object is in orbit. In order to assure such continuouscalibration with an active satellite system, it is necessary to transmitlight signals from the satellite day and night. It will be apparent thatthe limitations on power generation at the satellite make suchcontinuous signal emission impractical.

Optical tracking may be accomplished passively by us- ,ing reflected sunlight from the satellite body eliminating the requirement for a powersupply at the satellite. Unhappily such a system is limited to operationduring the period of twilight. In the daytime the noise level is toohigh and of course at night the sun is not reliably available. Anothermethod available involves an active optical tracking system with pulsedlight signals emitted from the satellite. Even an active system of thischaracter, however, is not useful in the daytime because of the highnoise level and limited power available. The operation of an activepulsed light emission system is essentially limited to use only duringthe earths shadow or at night.

An active system of this character really requires a significant powersupply to maintain a flashing light in operation. In contrast with theabove the present invention utilizes reference light sources from aremote reference source as, for example, a ground beacon search lighthaving a beam width of the order of one degree.

The present invention differs from the prior art in effectingmeasurements of the angle between the directions of a pair of referencesources at the satellite. In the prior art angle measurements arenormally made from the earth. Such measurements are subject todistortion from atmospheric refraction resulting in substantial errorsin the apparent angle measured. In accordance with the principles of thepresent invention, at the satellite direction angles can be measuredwith substantially reduced errors due to atmospheric refraction. Fornormal triangulation techniques, at least two angle measurements Incontrast, a single measurement effected at the satellite obviates therequirement for two separate angle measurements. Furthermore, forextreme accuracy, such measurements are incapable of being madesimultaneously. Since the present invention reduces by at least one-halfthe number of simultaneous angle measurements required,

timing error are reduced and time coincidence assured.

Because the angle between the directions of the refer ence sources aremeasured directly at the satellite, only approximate stabilization isrequired in contrast with the prior art system wherein a high degree ofsuch stabilization at the satellite is a necessity.

Where the angle measurements are made on earth, timing errors relatingto transmission delays and the inability to coincide the measurementwith true time at the satellite introduce further errors. Incontradistinction the present invention contemplates a simple anglemeasurement effective at the satellite. Since a time indicator iscarried at the satellite timing errors are substantially eliminated orat least reduced to the limits of modern time measurement accuracy.

The tracking system of the present invention, again in contrast withprior art optical systems, is useful twentyfour hours a day. While theuse of the visible light section of the electro-magnetic radiationfrequency spectrum is contemplated herein, signal to noise ratio in thepresent system is substantially enhanced by the use of sharply selectedfrequencies, e.g., monochromatic light, and modulation of thetransmitted light, e.g., interrupting the light signals in a desiredpattern.

Thus the system of the present invention realizes the full advantage oftransmitting signals from the ground where unlimited power supplycapability is available. The signal to noise ratio is substantiallyenhanced at the satel lite in the light of power, frequency, andmodulation or coding discrimination at the satellite. Angle measurementsand time indications are taken at the satellite to provide a systemwhich optimizes ground support and satellite equipment.

It is, therefore, an object of the invention to provide an improvedobject tracking system providing a high degree of accuracy.

It is a further object of the invention to provide an improved trackingsystem for producing indications of object position with high accuracy.

Yet another object of the invention is to provide an improved trackingsystem providing indications of the time at which measurements are takenwith improved accuracy.

Still another object of the invention is to provide an v improvedtracking system capable of simultaneously providing an angle measurementrelating to the positions of a pair of reference sources.

A still further object of the invention is to provide an improvedtracking system capable of producing position indications with a highdegree of structural simplicity.

A further object of the invention is to provide an improved trackingsystem exhibiting reduced errors due to atmospheric conditions.

A still further object of the invention is to provide an improvedtracking system requiring no tracking signal power supply at the object.

Still another object of the invention is to provide an improvedsatellite body tracking system wherein a lesser degree of stabilizationof the satellite body is required while providing improved accuracy inmeasurement.

Yet another object of the invention is to provide a satellite bodytracking system exhibiting an improved signal-to-noise ratio for thetracking signal at the satellite body.

A further object of the invention is to provide an improved satellitebody tracking system that is continuously operable.

In accordance with the invention there is provided a tracking system.The tracking system includes an object to be tracked. A furtherdirection-sensing means is coupled to the object for sensing thedirection of a first remote reference source in response to its signal.A sec ond direction sensing means is coupled to the object for thedirections of the reference sources.

sensing the direction of a second reference source in response to itssignal. The first and second sources are spaced a predetermined distanceapart. Angle means are coupled to the first and second direction-sensingmeans for measuring the angle between the direction of the first andsecond reference sources to provide an indication of a selectedposition-characteristic of the object.

In one form of the invention a satellite tracking system is provided fora satellite body in motion in an orbit in space. Direction-sensing meansare carried by the body.

In another form of the invention an optical directionsensing means isused as, e.g., a telescope.

In one embodiment of the invention a time-indicating means is coupled tothe object to provide an indication I of time at which the angle ismeasured.

In another embodiment of the invention data transmission means arecoupled to the angle means for transmitting angle data to a dataprocessing means.

In still another embodiment of the invention the radiant signal from aremote reference source is radiated at a selected frequency to enablefrequency discrimlnation of the signal.

In another form of the invention the tracking signal is modulated as,for example, by interrupting the signal in a predetermined or desiredpattern.

In still another form of the invention orientation means are coupled tothe direction-sensing means for aligning each of the direction-sensingmeans with its corresponding source reference. In operation, e.g., theorientation means may also respond to an err-or signal produced bynon-alignment of the reference source and the directionsensing means.

In accordance with one form of the invention three direction-sensingmeans are provided coupled to the object for sensing the directions ofthree reference sources in response to the respective signal of eachsource. The

measurement of the three angles between the directions of each pair ofreference sources completely defines the position of the object to betracked.

In the preferred embodiment of the invention the angle means includeelectro-mechanical means coupled to the direction-sensing means formeasuring the angle between The electro-mechanical means may be, e.g., apotentiometer or a shaft angle encoder.

Further in accordance with the invention is provided the method ofproviding a position indication of a body moving in space. In accordancewith the method the directions of a plurality of remote referencesources are sensed from the body in response to a respective signal fromeach of the sources. The sources are spaced predetermined distancesapart. The angle between the directions of a pair of sources is measuredsuccessively. The

position-characteristic of the body is computed from the Y successiveangle measurements in combination with selected data relating to thesources. -In a modification of the method the angle between thedirections of each of. a plurality of pairs of reference sources aremeasured. The position-characteristic of a body is then computed fromthe angle measurements in combination with selected data relating to thesources.

For a better understanding of the present invention, together with otherand further objects thereof, reference is made to the followingdescription taken in connection with the accompanying drawings and itsscope will be pointed out in the appended claims.

In the drawings: 7

FIG. 1 is a schematic diagram of a satellite body tracking systemembodying the invention and illustrating certain principles of theinvention;

FIG. 2 is a schematic diagram of a part of the system in FIG. 1 andillustrates a further aspect of the principles of the invention;

FIG. 2A is schematic diagram illustrating another aspect of theprinciples of the invention;

v are known. In the past it has been considered essential FIG, 3 is aschematic diagram of a satellite body embodying the tracking system ofthe invention;

FIG. 4 is a schematic diagram illustrating a modification of the systemin FIG. 1;

'FIG. 5 is a schematic block diagram broadly illustrating a satellitetracking system embodying the invention;

FIG. 6 is a detailed block diagram illustrating a preferred embodimentof the invention;

FIG. 7 is a schematic block diagram illustrating a modifiea'tion of thetracking system in FIG. 6;

FIG. 8 is a side view of a direction-sensing means embodying theinvention;

FIG. 9 is a face view of a camera used in accordance with the presentinvention;

FIG. 10 is a partially schematic, perspective view of a camera used inaccordance with the present invention.

In general the present invention comprises a novel method and apparatusfor locating precisely the position of an artificial satellite withrespect to an orbital body.

In a preferred embodiment of the invention the orbited body, the earthfor example, is provided with at least a pair of spaced beacons andpreferably networks of beacons distributed a predetermined distanceapart about the earth generally along the satellites orbital path. Ateam consisting of two or three beacons is adapted, by means ofauxiliary tracking equipment, to follow the satellite and illuminate itwith a narrow beam of monochromatic light. In accordance with theinvention the satellite is equipped with a plurality of gimbaled lightdetecting devices each adapted to lock on and track separately aparticul-ar beacon that is being monitored. The angle between eachdetecting device is measured at a recorded point in time and this datasent to a data processing system, which may be located on earth and/ orin the satellite and which is adapted to provide correcting signals tothe detecting devices and provide the data for plotting the satellitesposition.

Principles of operation The principles of operation will now bediscussed with reference to the drawings and with particular referenceto FIGS. 1 and 2. As shown in FIG. 1 a satellite 101 is in motion aroundthe earth, generally indicated at 102, in an orbit 103 indicated by adashed line. A plurality of reference sources or ground beacons 104,105, 106, 107, 108 and 109 are shown. The beacons are also indicated asB B B B B B The actual number of beacons in use are chosen in such amanner as to provide continuous illumination of the satellite 101. Thesatellite, as shown, is indicated for its position at various T A pairof directionsensing means 110 and 111 are shown coupled to the satellitein the positions relating to times T T and T The beacons B B etc.provide a tracking signal beam 112 to illuminate the satellite. For eachposition at the satellite 101 at least one triangle is determinedbetween it and a pair of reference sources. As shown, a pair ofreference sources, e.g., B and B provide a base line of known length,i.e., the reference sources are spaced a predetermined distance apart.The distance D between the sources 104 and 105, for example, may be 1000or 1200 miles. The angles ,6 and ,8 are the direction angles measuredfrom the base line B -B to the satellite 101 for the as measured fromthe satellite of the direction lines to a 7 pair of sources.

In accordance with the principles of triangulation a point in a planemay be determined when the distance between two reference pointsis knownand the direction angle from each of the reference points to a thirdpoint to utilize this basic information to determine the position of thesatellite in a known orbit at a given time. Ordinarily the plane of theorbit is known or predetermined at the time the tracking problem arises.When, however, it

is desired to define the location of the satellite at a given time withrespect to an orbit which has been predetermined within a given degreeof tolerance, angle a adequately and uniquely defines the satelliteposition. The measurement of the angle a can be obtained at thesatellite 101 by the use of direction-sensing means 110 and 111. In theoptical situation such direction sensing means may be a pair oftelescopes so gimballed as to permit independent orientation orpositioning of each telescope in alignment with the direction of areference signal source such as a beacon signal light. The beacon 104,105, etc. may be a powerful Searchlight providing a beam width ofone-half to one degree. For tracking purposes the beacon, e.g., Brotates the light beam through the angle [3 to follow the satellite. Thesignal 112 derived from the beacon 104 broadly defines the location ofthe satellite 101 when the satellite is properly illuminated.

As will be more completely discussed below, the orientation of thebeacon relative to the satellite provides direction data which istelemetered to the satellite and commands a direction-sensing means toalign itself with that beacon. Once the direction-sensing means andcorresponding reference sources are in alignment, tracking isaccomplished by controlling the respective motions of the referencesource and its corresponding direction-sensing means to maintainalignment.

A single measurement of the angle on at a given time presents anindication of a selected position-characteristic of the satellite atthat time. By taking successsive measurements in time it is thenpossible to determine precisely the character of the orbit. It will beapparent from the above that a single measurement of the angle a is muchto be preferred over two separate measurements of direction angles takenfrom each beacon. The precise time coincidence of measurement, e.g., ofB and 5 relating to beacons B and B respectively, is impossible, whereastime coincidence of the measurement of angle a is inherent since only asingle measurement is involved. A single measurement of angle a definesthe satellite position uniquely with respect to a known orbit.Successive measurements of angle a provide adequate information for anaccurate description of a particular orbit.

As the satellite moves along its orbit the particular sources chosen fortracking change. The telescope 110, e.g., may be aligned with B thenwith B B B etc. in a given locale, the satellite may use two of a numberof available sources. At times T and T the satellite may use B and B orB and B or B and B Referring now to FIG. 2 there is here presented a diagram illustrating the planar case for a portion of the motion of thesatellite 101. In FIG. 2, R is the geocentric satellite radius; R is theradius of the earth 102; angles 6 and ,8 are the same as in FIG. 1,angle 0 is the angle between geocentric radii of the beacons B and B Cis the earths center; and h is the satellite height above the earthssurface. For angle a varying from approximately 40 to 60, angle 6 isapproximately 16 of are, ,8 varies from approximately 100 to 40 and 13varies from approximately 40 to 100. The angle 0: reaches a maximum whenthe satellite is equidistant from the beacons B and B In the extremecase angle on becomes 0 when the satellite and beacons are colinear. Thedifierence between the beacon direction angles, {3 ,B is inverselyproportional to the angle 0:.

It would appear from, e.g., the law of sines, knowing one side and anopposite angle is insufiicient unambiguously to describe a uniquetriangle. A moving body, however, generates other curves providinguseful data.

If, e.g., the angle a were a constant, motion of the satellite wouldgenerate a curve relative to B and B which is unique. For each such oranother curve would be generated.

Thus, when successive measurements are taken with respect to time, thenecessary data is produced uniquely to define the satellite position.

Referring now to FIG. 2A, there is here presented a schematic diagramillustrating the difference in the apparent error of the beacondirection angle as observed from earth as opposed to the apparent errorviewed from the satellite S.

In FIG. 2A triangles S B B and S B B present the true positions of thesatellite and beacons and the direction lines between them. Thetriangles S B B and S B B present the apparent positions and directionlines when observed from the ground.

The angles 9 and 6,, illustrate the apparent incremental errors indirection angles associated with satellite position S as observed fromthe beacon stations B and B respectively. The angles 111,, and ,cillustrate the corresponding apparent incremental errors as observedfrom the satellite S The angle er is the true angle between the beacondirections from the satellite S the angle 0: is the apparent angleobserved at the satellite. For the position S and apparent position Sthe corresponding angles are a b be b b and b" Although the portions arenot accurate as drawn, the reason that the angular measurement from thesatellite is subject to less error than that measured from the groundwill be apparent from FIG. 2A. The height h,,, of the atmosphere whereineifective refraction takes place is considered to be 50 miles. For a1000 mile satellite h the distance that the signal travels in anunrefracting environment is 950 miles; that is, the height of thesatellite above the atmosphere is 950 miles. Thus the error anglesubtended with respect to the satellite is extremely small relative tothat angle subtended with respect to a ground beacon. It turns out thatfor 0 in the order of 100" of arc, is only 2.5" of arc.

The error in a for S is only 5" whereas the error in {3 and 9 additivelyis 200" of arc.

Moreover, for S where B the very character of the measurement of theangle a substantially reduces even this small error. The true angle a asshown, diifers from the apparent angle a very little since themeasurement compensate for the errors to a large degree; the net error au being Conversely the errors measured from the ground are additive.Thus, in triangle S 'B B the apparent satellite position S is verydifferent from its true position indicated at S For purposes ofillustration, the curvature of the earth and atmosphere are ignoredherein.

Description and operation of the tracking system in FIG. 3

Referring now to FIG. 3 there is here illustrated a satellite trackingsystem embodying the invention. A satellite body 200 supports a pair oftelescopes 201 and 202. The telescopes are aligned as shown with a pairof reference sources of optical signals B and B The telescopes arespherically gimbaled with a pair of spherical supports 203 and 204 whichmate with spherical mounts 205 and 206 respectively. So gimbaled thetelescopes may be rotated arcuately about a pair of orthogonal axes x--xand y-y (FIG. 6) for astronomical azimuth and inclination positioning.The two degrees of freedom thus provided by the spherical gimbaling may,as is well known in the art, be supplied by other suitable axis supportdevices, such as yoke and axle devices. The mechanical means forengaging the telescopes for alignment purposes is not shown. Themeasurement of the angle a is obtained electro-mechanically with apotentiometer generally indicated at 207.' The potentiometer circuitcomprises a stator resistor 208, a source of power 209, here indicatedas a battery direct current supply, and a movable connection tap 210coupled to a mechanical positioning arm 211. The voltage along theresistor 208 varies in proportion to the angle a.

The arm 211 is connected to the telescope 201 and moves arcuately. Themovable tap 210 provides an electrical connection to a data link 212which transmits the information to a data processing center, e.g.,'onearth. The time at which the measurement is taken is provided with atime indicator 213 which is coupled to the data link 212. The data linkboth transmits and receives data for control and processing purposes. Asshown the data link is coupled to an orientation control 214 and asecond orientation control 215 which are in turn coupled to thetelescopes T and T respectively.

Broadly the direction angle of the satellite as determined by theangular positions of the beacons in the ground reference station andthis information is transmitted by the data link 212 to the orientationcontrol 214 for T; which in turn aligns the telescope T with the groundbeacon B Similarly a command signal from the ground beacon B istransmitted through data link 212 to the orientation control 215 whichis coupled to the telescope T for alignment thereof.

Description and operation of the tracking system in FIG. 4

A tetrahedron is fully described by its base lines and apex angles.Since the beacons are prelocated and spaced predetermined distancesapart, the base lines B 8 B B;,

.and B B are known. The apex angles a a and :1 are measured at thesatellite to fulfill the required information for explicitly determiningthe position of the satel- .lite S.

Referring now to FIG. 4 there is here illustrated a schematic diagram ofa tracking system involving the use of more than two ground referencesources or beacon stations. Three reference sources B B and B areprovided for use in aligning the position of the satellite S. Here theconcept of the intersection of three planes is utilized to provide fullposition information by measurements simultaneously made in time. Thusthe angle (1 between B and B provides an indication of thepositioncharacteristic of the satellite in the plane defined by triangleSB B In like manner angle a provides a position indication of thesatellite in the plane SB B and the angle a provides a positionindication of the satellite in the plane SB B It will be apparent fromthe above that the satellite together with the beacon station B B and Bpresent a system of intersection points describing a tetrahedron. Theintersection of the planes of the tetrahedron, common apex of the threetriangles, occurs at the position of the satellite. Here again themeasurement is made at the satellite of the angle between the directionsof the beacons. Here we see that three angles measurements provide thenecessary data as to the position of the satellite as opposed to six aswould otherwise be required from the ground.

Description and operation the tracking system in FIG. 5

Referring now to FIG. 5 there is here broadly illustrated a completesystem for satellite tracking and control with its supporting datalinks. The devices located on earth are shown to the left of the dottedline; those devices carried by the satellite are shown to the right ofthe dotted line. Reference signal sources which are the beacons on earthare directed toward the satellite and coupled through thedirection-sensing means including the telescopes and the satellitetracking system. Data as to the direction angles (cg. [3) of thereference signal sources on earth are coupled to a data processing meanslocated on the earth. The data processing means is in turn coupled to adata link. Radio or other radiation communication is established betweenthe data link on earth and a data link carried by the satellite.Measurements of the angle a at the satellite between the directions of apair of reference electro-magnetic signal sources such as beams from thebeacons B B are coarsely made electro-mechanically. Data as to theposition of the direction-sensing means at the satellite and the angle ameasurements are transmitted to a data processing'means carried by thesatellite.

Thus a system of reference signal sources 501 represents singals frombeacons B B B to a directionsensing and tracking system 502 carried bythe satellite. The signal sources are coupled to a data processing means503 located on earth. The earth data processing means is in turn coupledto an earth data link 504 in communication with a satellite data link505. Data is transmitted and received by both data links 504 and 505.The coarse measurement of the angle a. is provided by anelectro-mechanical angle measurement means 506 such as the potentiometercircuit of FIG. 3. The coarse angle measurement means is coupled to adata processing means 507 carried by the satellite. Thedirection-sensing and tracking system is coupled to a fine measurementmeans 508 for Act which is in turn coupled to'the data processing means507. Time indicator 509 is coupled to the data processing means 507 andthe satellite data link 505.

As the satellite moves in its orbit, individual beacon sations B B Bprovide optical signals which are received by the direction-sensing andtracking system at the satellite. The direction-sensing system may alignitself with the individual beacons by virtue of a beam sensing mechanismcarried by the satellite which automatically centers thedirection-sensing means with respect to the signal from the referencesignal source. Because of frequent requirement for extreme accuracy, amechanism for accurately measuring the angle on is required. Theelectro-mechanical means 506 may be adequate for measuring the angle orwithin an accuracy, e.g., in the order of minutes of arc. However, whenaccuracies in the order of seconds of are are required, an additionalessentially micrometer measurement device is required ofiering anexpanded base.

Description and operation of the tracking system in FIG. 6

Referring now to FIG. 6, there is here illustrated a detailed schematicblock diagram of a satellite tracking system carried by the satellitedesigned for use with a pair of telescopes cooperating with a pair ofreference sources located at the ground. The system as here illustratedgenerally presents a pair of telescopes, a position orientation systemfor each of the telescopes and an optical signal receiving andprocessing system. As shown a data processing means and data linktransmitting and receiving means are coupled to a time indicator forproviding an indication of time at which data is processed or receivedor transmitted.

Thus a first-direction-sensing means or optical telescope 601, hereindicated as T is coupled to a Y axis drive means 602 which in turn iscoupled to a Y axis servo 603. An X axis drive means 604 is coupled tothe scope 601 and an X axis servo 605 is coupled to the drive means 604.A second direction-sensing means, a telescope 600 here alternatelyindicated as T is coupled to a Y axis drive means 606 and X axis drivemeans 607. The drive means 606 is coupled to a Y axis servo control 608and the drive means 607 is coupled to an X axis servo control means 609for the telescope T Both telescopes are coupled to a measurement means610. The telescopes 601 and 600 are optically coupled to a photoelectricprocessing system involving an optical filter means 611, coupled to aphotoelectric converter 612 which is in turn coupled to a demodulator613. The telescopes, the servos, the measurement means and thedemodulator are all coupled to a data processing means 614 which is inturn coupled to a data link means 615, time indicator means 616 coupledto the data processing and data link means 614 and 615.

The operation of the direction-sensing orientation control system willbe described with particular reference to the T telescope system. Aground reference source or optical beacon station B directs a light beamat the satellite. At the same time instructions from the referencestation B are communicated via the satellite data link 615 and dataprocessing means 614 to provide initial X and Y axes orientation data.Since the actual satellite position orientation of T is stored in thedata processing means 614, a command signal from the ground station iscompared in means 614 with the position of the telescope. An errorsignal is produced providing specific orientation instructions for the Xand Y axes. An error in the Y axis is coupled from the data processingmeans 614 to the T Y axis servo 603 which in turn controls the T Y axesdrive mechanism to control the axes position of the scope T Theorientation of the telescope T is independently controllable by rotatingthe telescope along orthogonal arcuately reference curves about the Xand Y axes. The telescope T thus has two degrees of freedom to enableits alignment with the signal emitted by a ground reference station. Thealignment of the telescope T may be independently tracked. A discrepancybetween the actual orientation of T is compared with data from B as toits XY orientation. Any resulting error signal will appear in the dataprocessing means 614. Alternatively, by using a beam center sensingdevice carried by the satellite and operative to produce an error signalwhen T is misaligned with respect to the center of the beam, T can becaused to track the beam from B automatically.

In a similar manner the orientation control system coupled to the secondtelescope T or 600 operates to position it with respect to a pair oforthogonal XY axes to provide it with two degrees of freedom. Thus anerror signal emanating from the data processing means 614 may applied tothe T Y axis servo 608 and coupled to the T Y axis drive mechanism whichorient the telescope T with the respect to the Y axis. Orientation alongan orthogonal X axis is produced by an error signal emanating from thedata processing means 614 and coupled to the T X axis servo 609, theservo is coupled to the T X axis drive mechanism 607 to control the Xaxis position orientation of the telescope T The drive means and thegimbaling of the telescope may assume any number of well knownmechanical arrangements. They may be coupled for example by acombination of azimuth and inclination yoke and axle couplings toprovide the two degrees of freedom. Positioning may be accomplished bysuitably gearing the XY axes axles or shafts to a drive motor.

Once the telescopes have been aligned with their corresponding referencesources the angle between them, angle or, is measured to provide anindication of the position of the satellite. This measurement istypically made by a potentiometer device such as described above or adigital shaft encoder measurement device of the type manufactured byRCA. The fine measurement of alpha may be accomplished by electronicmeans utilizing a photoelectric converter such as a television camera.

In order to provide operation for twenty-four hours a day it isnecessary to increase the apparent signal to noise ratio of the desiredsignal arising at the satellite. This problem is particularly intenseduring the daytime when the light emitted by the earth provides a veryhigh noise background. The desired system for discrimination may takethe form of frequency selection and modulation of a signal. Thus achoice of monochromatic light for the beacons as provided by a sodiumarc lamp or a mercury arc lamp cuts down the area of the visiblespectrum at which the satellite is illuminated. Furthermore, the lightsignal may be modulatedby flashing the light at a suitable rate. At thesatellite the light signals received by the telescopes T and T may bedirected from an optical filter means 611 to a light direction sensitivephotoelectric converter 612. The position of the point of impingement ofthe directed light signal provides a mechanism for .measuring the anglealpha. This mechanism for example could involve a -10 reference XY axisand a requirement that the points of impingement maintain a certainrelationship with, e.g., an arbitrary axis. It is possible tosuperimpose the light signals and require a nulling effect. Anydeviation from the null would indicate a misalignment with the beaconstations which provides an error signal to correct the appropriatetelescope.

Light energy applied to a photoelectric converter provides an indicationof a presence of a true signal as well as the mechanism for convertingthe data in the light signal into an electrical form for use for controlpurposes. Thus a signal of monochromatic light passing through thefilter 611 is converted by the converter 612 into an intermediatefrequency electrical signal which can be demodulated to provide anindication of the character of the signal.

The system as shown does not incorporate Doppler frequencydiscrimination.

An intermediate frequency signal obtained by e.g. heterodyning areference signal, not shown, with the converted optical signal may becoupled to the data processing means 614 to be compared with storedreference data indicating the presence or lack of presence of a signal.Given an indication of a true signal received, the tracking system uponcommand from the data processing means locks on the correspondingbeacon, e.g., B and tracks it continuously. At the same time theindication of acquisition of B is transmitted by the data link 615 tothe ground data link system and there introduced to a central dataprocessing center such as indicated above with reference to FIG. 5. Thetime indicator means 616, which may be an extremely accurate mechanismsuch as an atomic clock, provides time indications to the dataprocessing means 614 and the data link 615. All data with respect toposition or orientation or acquisition is thus referenced in time bymeans of a self contained time indicator.

The data processing means 614 receives, processes and stores data withrespect to the orientation of the telescopes, the measurement of angle aand time as well as the characteristics of the optical signal receivedat the satellite. The extent of data processing accomplished at thesatellite is determined by optimizing available data transmissionbandwidth relative to complexity of mechanization required at thesatellite. In any case, data as to the orientation of the telescopeswith respect to the XY axes, the acquisition of the beacon station, thetime indications, and the angle on measurements may be transmitted viathe data link 615 to a ground central data processing center.

The servo systems used for positioning the telescopes include a feedback loop from the telescopes as to their position with references tothe XY axes. This information is relayed back to the servo controls603,605, 608, 609 in accordance with well known feed back techniques.

Description and operation of the tracking system in FIG. 7

, Referring now to FIG. 7 there is here illustrated a tracking systemcarried by the satellite for use in conjunction with a modified trackingsystem such as that illustrated diagrammatically in FIG. 4. As shownherein three telescopes are used corresponding with three beaconstations on earth B B and B A system is provided for measnring the threeangles between the directions of each pair of beacon stations. Theoptical signals are directed by telescopes to a photoelectric conversionand filter means and demodulated data storage system. An orientationcontrol system for XY positioning of the telescopes T T and T is coupledwith a data processing system, which, in turn, is coupled to a data linksystem for transmitting and receiving appropriate information.

Thus there are three telescopes 701, 702, and 703 which are coupled inpairs as shown to three alpha angle measuring devices 704, 705, and 706.The alpha measurement devices present means for measuring the anglebetween the reference stations B and B X the angle between referencestations B and B X and the angle between the reference stations B and BX The telescopes are coupled to an orientation control 707 whichcontrols the orientation with respect to the X and Y axes of thetelescopes T T and T in a manner similar to that described with respectto FIG. 6. The telescopes are optically coupled to a phtoelectricconverter and filter 708 which in turn is coupled to a demodulator 709.The photoelectric conversion system operates in a manner similar to thatdescribed with respect to FIG. 6. Demodulator 709, the telescopes andthe orientation control devices, are all coupled .to a data processingmeans 710 which is in turn coupled to a data link transmit and receivemeans 711. Again a time indicator means 712 is coupled to data link 711and the data storage means 710. The system as presented in FIG. 7 issomewhat more complicated than in FIG. 6 in that three tracking systemsare required for the telescopes plus two additional angle measuringdevices for measuring the direction angles with respect to an additionalreference beacon station. The function of computing the actual values ofangles X X and X may be accomplished either at the satellite or at acentral data processing system on earth.

Description and operation ofia modified telescope in FIG. 8

Referring now to FIG. 8 there is here illustrated a telescope modifiedfor use in the present invention to avoid the requirement for twoindependently controlled telescopes. A telescope 715 is coupled to anoptical flat 716 'which is rotatable about a shaft or axle 717 providingan axis perpendicular to the plane of the drawing. Surrounding thetelescope is a rotatable tube 718 which is rotatable about the opticalaxis 719 of the telescope. The tube 718 provides a rotatable support fora yoke 720 which supports the optical flat 716 at the opposite ends ofits axis of rotation. The optical flat is carried by the hinge 717 insuch .a manner that it is rotatable about an axis extending through thehinge perpendicular to the plane of the drawing. It will be apparentthat the combination of rotations thus available provides two degrees offreedom for the telescope.

In operation the telescope 717 is aligned with its respective source ofsignal e.g. a ground beacon station B which directs an optical signalthrough the flat 716 along the optical axis of the telescope. A signalderived from a reference ground beacon B arrives at a direction at anangle alpha with respect to the direction angle of the reference groundbeacon B It will be apparent that the axes through the hinge 717 andalong the optical axis of the telescope are orthogonal. The rotation ofthe optical flat about the axis of the hinge 717 provides a measure ofonehalf of the angle alpha. This rotation can be correlated by means ofa differential transformer, rotary potentiometer, digital shaft encoderor other electro-mechanical conversion device to produce an electricsignal proportional to the angle a.

An orientation control means may operate a servo positioning controlcoupled to the optical flat to position it in accordance with commandinstructions from its corresponding reference ground beacon B The tube718 is rotated about the optical axis to provide alignment of theoptical signal B with the telescope 715. Such an arrangement has adefinite advantage over independently controlled telescopes in thatoptical errors produced by misalignments within the telescopes areeliminated. Here the signals from both beacons are directed through thesame telescope. This also enables a ready null adjustment and greatlysimplifies the structure required for direction-sensing. Here a singleapparatus provides direction-sensing means for both beacons. Thisconcept can of course be further enhanced with a third reflectingsurface transparent-refiecting device which is oriented with respect toa third beacon.

Description and operation of the camera illustrated in FIGS. 9 and 10Referring now to FIGS. 9 and 10 is here illustrated a television camera722 useful for electronically measuring the angle a between thedirections from the satellite of a pair of beacons. In accordance withwell known television camera techniques the positions of optical signalsreceived from reference ground stations B and B may be determined withrespect to a predetermined system of reference X and Y axes. Here theprinciple of utilizing a relatively fast moving scanning electron beamto measure the distance between the received points provides anextremely fine measurement of the angle a. This system is preferablyused in conjunction with .a coarse measuring device which iselectro-rnechanical in character. As shown in FIG. 9 a light signalrepresentative of B appears on a screen 721 in a very definite positionwith respect to a scanning beam. This position is located with respectto a pair of XY reference axes. The measurement is made when the twoindications from the signals from B and B are linearly arranged on the Xaxis. The separation between the signal indications B and B representthe measurement of the angle Au. In addition, misalignment errorsbetween the telescope T and its corresponding ground reference beacon Bappear as displacements on the screen as AY or AX Similarlymisalignments of the telescope T with its corresponding referencestation B appear on the screen as AX or AY This information, as shown inFIG. 10, is relayed via a data processing means 723 to the appropriateorientation control to correct the misalignment. The distance X X of theindications of the beacon signals indeed provides a measore of the anglea.

Motion of the B signal with respect to reference X when accompanied by asimilar motion of the B signal, produces an error signal forrepositioning for both telescopes. When the telescope T becomesmisaligned with the beacon B this information is stored in a dataprocessing means. The exact amount of displacement can then be measuredby the scanning beam in the television circuit.

The background interference encountered in tracking an object on theground from the satellite consists of generally a brightness variationfrom the ground due to shadows and variations of reflectivity of groundobjects. In some cases the contrast between these objects may be as muchas percent for certain wave length regions. The maximum brightness ofthe scene, e.g., bright sunlight may be derived from the solar constant,luminous efliciency of sunlight and the diffuse reflectivity of snow. Itturns out to be approximately three lumens/cm. /steradian.

Maximum spacial discrimination may be realized when the discriminationpattern of the tracker is matched to the apparent size of the source.Thus the element size of the discrimination pattern is of the order ofone second are or approximately 2.5 X 10* steradian, the illuminancereceived from one element of maximum background of threelumens/cmP/steradian is a little less than 10 lumens/ cm. which is aboutthat received from a first magnitude star.

The use of an electronic photoelectric converter for effecting the finemeasurement of angle a greatly enhances the spacial discrimination ofthe beacon by virtue of the display presented. The beacons that may beused in conjunction with the above preferably utilize a carbon arcSearchlight source working with a photo tube as a receiver. Althoughinfra red detectors are useful for the present application, visible wavelengths yield greater intensity. A beacon Which is useful comprises afive foot diameter Searchlight with a beam width approximately one-halfdegree at about 10 beam candle power. At 1000 miles such a sourcesubtends an angular size less than one second of are as seen from asatellite at 1000 miles. The source may be modulated in frequency at asuitable audio frequency. From the preliminary orbital data, the

beam is directed continuously at the satellite. The apparent position ofthe searchlight and the telescopes field of view is accuratelydetermined by a television scan device as described above. This positiondata is stored together with telescope gimbal position data.Simultaneously similar tracking data from other ground stations arestored. Resulting triangulation data, sutlicient for an accuratedetermination of position by triangulation techniques, are transmittedto the ground after processing.

It will be apparent that the present invention greatly enhances the artof tracking of moving objects. The present invention has greatapplication in the many areas where object location and positioncharacteristics of moving objects are of interest.

While there have been described What are at present considered to be thepreferred embodiments of the invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is, therefore,aimed in the appended claims to cover all such changes, modifications asfollow in the true scope and spirit of the invention.

What is claimed is:

1. A system for locating an artificial satellite in orbit about aheavenly body, comprising a plurality of signal generating stationslocated in spaced measured relation on the surface of said body, atleast a pair of signal sensing elements mounted to said satellite, saidelements being angularly movable relative to one another, means foraligning each of said elements with separate ones of said stations,means located in said satellite for measuring the angle between saidelements, means for converting said angle into electrical informationand means for transmitting said information to said heavenly body.

2. A system for locating an artificial satellite in orbit about aheavenly body, comprising a plurality of light beam generating stationslocated in spaced measured relation on the surface of said body, atleast a pair of beam sensing elements mounted to said satellite, saidelements being angularly movable relative to one another, servo meansfor aligning each of said elements with separate ones of said stations,means located in said satellite for measuring the angle between saidelements, analogue means for converting said angle into electricalinformation and telemetering means for transmitting said information tosaid heavenly body.

3. A system for locating an artificial satellite in orbit about aheavenly body, comprising a plurality of light beam generating stationslocated in spaced measured relation on the surface of said body, atleast a pair of beam sensing elements mounted to said satellite, saidelements being angularly movable relative to one another, servo meansfor precisely aligning each of said elements with separate ones of saidstations, means located in said satellite for measuring the anglebetween said elements, analogue means for converting said angle into anelectrical signal, timing means for indicating the time of anglemeasurement and means for transmitting said signal to said heavenlybody.

4. A system for locating an artificial satellite in orbit about aheavenly body, comprising at least a pair of light beam generatingstations located in spaced relation on the surface of said body, atleast a pair of light beam sensing elements mounted to said satellite,said elements being angularly movable relative to one another, a servoloop for aligning each of said elements with separate ones of saidstations, means located in said satellite for measuring the anglebetween said elements, analog means for converting said angle into anelectrical signal, means for timing said measurement, telemetering meansfor transmitting said signal to said heavenly body, and means located onsaid heavenly body for receiving and decoding said signal.

References Cited by the Examiner UNITED STATES PATENTS 2,489,219 11/49Herbold. 2,569,328 9/51 Omberg. 2,715,995 8/55 Wirkler. 2,982,958 5/61Yulo 343-412 X CHESTER L. JUSTUS, Primary Examiner.

FREDERICK M. STRADER, KATHLEEN CLAFFY,

Exam e

1. A SYSTEM FOR LOCATING AN ARTIFICIAL SATELLITE IN ORBIT ABOUT AHEAVENLY BODY, COMPRISING A PLURALITY OF SIGNAL GENERATING STATIONSLOCATED IN SPACED MEASURED RELATION ON THE SURFACE OF SAID BODY, ATLEAST A PAIR OF SIGNAL SENSING ELEMENTS MOUNTED TO SAID SATELLITE, SAIDELEMENTS BEING ANGULARLY MOVABLE RELATIVE TO ONE ANOTHER, MEANS FORALIGNING EACH OF SAID ELEMENTS WITH SEPARATE ONES OF SAID STATIONS,MEANS LOCATED IN SAID SATELLITE FOR MEASURING THE ANGLE BETWEEN SAIDELEMENTS, MEANS FOR CONVERTING SAID ANGLE INTO ELECTRICAL INFORMATIONAND MEANS FOR TRANSMITTING SAID INFORMATION TO SAID HEAVENLY BODY.